This invention relates to an optical glass and, more particularly, to a high-refractive, high-dispersion optical glass having optical constants of a refractive index (nd) within a range from 1.78 to 1.90 and an Abbe number (νd) within a range from 18–27, having excellent light transmittance and resistance to devitrification, and having excellent internal quality of glass.
If it is desired to produce a high-internal quality glass with an excellent yield in manufacture of glass, it is a general practice to employ a melting apparatus in which at least a portion which comes in contact with molten glass is made of platinum or platinum alloy in all or a part of the portion. For example, a crucible, tank, stirring blade and shaft made of platinum or platinum alloy are often used. In this case, however, if the melting temperature becomes high or a melting hour becomes long, the amount of platinum which melts into the molten glass increases and, as a result, platinum ion which has melted into the glass absorbs light. This causes a tendency toward decreasing transmittance, particularly transmittance in the short wavelength region, of the glass.
On the other hand, since internal quality of an optical glass is evaluated by the degree of striae, bubbles and inclusion (e.g., fine crystals or bubbles due to devitrification or other causes) existing in the glass, a glass having poor resistance to devitrification, poor removal of foam or poor melting property is subjected to a higher melting temperature or a longer melting time to make the melting condition optimum or is subjected to a process of increasing a defoaming agent for reducing striae, bubbles and inclusion of the glass and thereby improving the internal quality of the glass. Such treatment of the glass, however, decreases transmittance of the glass for the above described reason, i.e., melting of platinum into the glass and for various other reasons including mixing of inclusion.
In an optical design, a lens made of a high-refractive, high-dispersion optical glass is used in combination with a lens made of a low-refractive, low dispersion optical glass for correcting chromatic aberration of an optical system. Such combination is used in various optical instruments. The higher the refractive index and dispersion of a glass become, the larger amount of ingredients which impart the high-refractive, high-dispersion characteristics to the glass must generally be added. Since, however, these ingredients mostly absorb light in a short-wavelength region, transmittance in the short-wavelength region of the glass tends to deteriorate. Further, as the refractive index of the glass increases, reflectance on the surface of the glass increases and, therefore, as the refractive index increases, transmittance tends to decrease also due to this factor.
Known in the art of a high-refractive, high-dispersion optical glass having good transmittance in the short-wavelength region is a lead silicate glass containing a large amount of lead. For example, Japanese Patent Application Laid-open Publication No. 57-34042 discloses a high-refractive, high-dispersion glass of a SiO2—PbO—B2O3 system containing a large amount of PbO which has excellent transmittance because the amount of molten platinum is small in this glass. The glass containing a large amount of PbO, however, is insufficient in chemical durability and, moreover, since it has a large specific gravity, a lens made of this glass is disadvantageous in producing an optical instrument of a light weight which is a rapidly prevailing tendency today. There is also a tendency toward ceasing use of a glass containing lead which is detrimental to the environment and, therefore, there is a demand for a high-refractive, high-dispersion optical glass which is free of lead and yet has excellent transmittance in the short-wavelength region which is equal to the SiO2—PbO—B2O3 glass.
As a method for evaluating transmittance of an optical glass, comparison of degree of coloring specified by JOGIS02-1975 is used. This method, however, cannot sufficiently evaluate color balance of transmitted light which is an important factor in the optical design.
For expressing the color balance of transmitted light, ISO Color Contribution Index (ISO/CCI (B/G/R)) is used according to which, by calculating this index on the basis of data of transmittance within a range from 350 nm to 680 nm measured by JOGIS02-1975, the color balance of glass bulk itself is evaluated.
The ISO Color Contribution Index of a high-refractive, high dispersion glass generally exhibits the characteristic that, in a glass bulk in which an average transmittance value within a range from 530 nm to 680 nm measured by JOGIS 02-1975 is substantially equal to that of a SiO2—PbO—B2O3 glass and photographic responses of the green sensitive layer and the red sensitive layer of an average color film are substantially equal to those of the SiO2—PbO—B2O3 glass, the higher the transmittance in the short-wavelength region of visible ray is, the smaller are values of G and R when the value B of the ISO Color Contribution Index is 0.
For example, commercially available PBH53W and PBH6W (made by Kabushiki Kaisha Ohara) which are typical PbO—SiO2 glasses containing a large amount of lead exhibit the characteristic that they have excellent transmittance in the short-wavelength region and excellent internal transmittance and, therefore, they have small values of G and R of the ISO Color Contribution Index.
As a high-refractive, high-dispersion optical glass which is free of PbO, Japanese Patent Application Laid-open Publication No. 54-112915 discloses a P2O5—RI2O and/or ZnO—Nb2O5 optical glass having very broad composition range and optical constants ranges. In this glass, however, glasses having a refractive index (nd) of 1.78 or over among specifically disclosed glasses are insufficient in resistance to devitrification and moreover insufficient in the melting property so that inclusion (fine crystals) tends to be produced in the glasses and, as a result, a glass of a good internal quality cannot be obtained. If a higher melting temperature or a longer melting time is employed for improving the internal quality of the glass, the amount of platinum which melts into the glass increases resulting in increase in values of G and R of the ISO Color Contribution Index of the glass bulk and, therefore, deterioration in the color balance in comparison with the PbO—SiO2 glass.
Japanese Patent Application Laid-open Publication No. 52-132012 discloses a P2O5—B2O—Nb2O5 and/or RIIO optical glass having very broad composition range and optical constants ranges. In this glass, however, PbO-free glasses having a refractive index (nd) of 1.78 or over among specifically disclosed glasses are insufficient in resistance to devitrification and moreover insufficient in the melting property so that inclusion (fine crystals) tends to be produced in the glasses and, as a result, a glass of a good internal quality cannot be obtained. If it is attempted to improve the internal quality of this glass, the color balance of the glass deteriorates in comparison with the PbO—SiO2 glass for the reason described above.
Japanese Patent Application Laid-open Publication No. 5-270853 discloses a SiO2—B2O3—P2O5—Nb2O5—Na2O and/or K2O optical glass having very broad optical constant ranges. In this glass, however, glasses having a refractive index (nd) of 1.78 or over among specifically disclosed glasses are so poor in transmittance in the short-wavelength region that values of G and R of the ISO Color Contribution Index of the glass bulk are large. That is, the color balance of the glass in comparison with the PbO—SiO2 glass is poor and the melting property of the glass is insufficient so that inclusion (i.e., fine crystals) and bubbles tend to be produced in the glass. If it is attempted to improve the internal quality of the glass, the color balance of the glass deteriorates even further for the reason stated above.
Japanese Patent Application Laid-open Publication No. 9-188540 discloses a P2O5—Nb2O5 optical glass. In this glass, however, glasses having a refractive index (nd) of 1.78 or over among specifically disclosed glasses have large values of G and R of the ISO Color Contribution Index of the glass bulk and, moreover, the melting property of the glass is insufficient so that inclusion (i.e., fine crystals) and bubbles tend to be produced in the glass. If it is attempted to improve the internal quality of the glass, the color balance of the glass deteriorates even further for the reason stated above. Japanese Patent Application Laid-open Publication No. 8-157231 discloses a P2O5—B2O3—Nb2O5—Li2O—Na2O—SiO2 optical glass and a P2O5—B2O3—Nb2O5—Li2O—WO3 optical glass. These glasses, however, are insufficient in removal of foam, are difficult to obtain a homogeneous glass and, moreover, absorb a relatively large amount of light in the short-wavelength region so that values of G and R of the ISO Color Contribution Index of the glass bulk are large in comparison with the PbO—SiO2 glass.
Japanese Patent Application Laid-open Publication No. 2001-58845 discloses a P2O5—Na2O—Nb2O5—Bi2O3 optical glass. Specifically disclosed glasses in this publication, however, are insufficient in removal of foam, are difficult to obtain a homogeneous glass and, moreover, absorb a relatively large amount of light in the short-wavelength region so that values of G and R of the ISO Color Contribution Index of the glass bulk are large in comparison with the PbO—SiO2 glass.
Japanese Patent Application Laid-open Publication No. 2002-173336 discloses a P2O5—Bi2O3—R′ optical glass. Specifically disclosed glasses in this publication, however, are insufficient in removal of foam, are difficult to obtain a homogeneous glass resulting in generation of fine crystals and, moreover, absorb a relatively large amount of light in the short-wavelength region so that values of G and R of the ISO Color Contribution Index of the glass bulk are large in comparison with the PbO—SiO2 glass.
Japanese Patent Application Laid-open Publication No. 2002-201041 discloses a P2O5—WO3 optical glass for precision pressing. This glass, however, has large absorption of light in the short-wavelength region. Moreover, specifically disclosed glasses in this publication are insufficient in removal of foam and are difficult to obtain a homogeneous glass. If a higher melting temperature or a longer melting time is employed for improving the internal quality of the glass, the amount of platinum which melts into the glass increases resulting in further increase in values of G and R of the ISO Color Contribution Index of the glass bulk.
It is, therefore, an object of the present invention to eliminate the above described disadvantages of the prior art optical glass and provide a high-refractive, high-dispersion optical glass having optical constants of refractive index (nd) within a range from 1.78 to 1.90 and Abbe number (νd) within a range from 18 to 27 which, while maintaining color balance which is equivalent to that of the SiO2—PbO glass, has excellent transmittance in the short-wavelength region, namely has small values of G and R of the ISO Color Contribution Index and excellent internal quality and resistance to devitrification.